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|Opticast Mirrors, Optical Replication, And Reality.
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Posted by Zack B./">Zack B. on December 29, 1999 00:50:13 UTC
Opticast mirrors are not spun cast, they are replicated using a negative mold.
This is done by first molding the plaster base as best you can. The optical surface is formed on the plaster base by putting a very thin layer of epoxy on the plaster and some release agent on a convex piece of glass with an optical quality surface on it (parabolic typically). You then press the two together and let it harden. Hopefully you can then separate them and viola!, a mirror!
I think that sometimes it takes two or more iterations of this to get decent optical quality. This is usually due to print through of irregularities on the plaster substrate. The print through occurs due to the shrinkage of the epoxy. For example, if you have a small pit on the plaster base, the epoxy will be deeper there, and there will be more shrinkage in the direction perpendicular to the surface, leaving a depression above it on the epoxy surface. If your epoxy shrinks 1% when it cures (all epoxy shrinks), then the irregularities on the plaster substrate will print through to 1% of their size on the optical surface. So you need to get the best possible surface on your substrate.
I have read a few technical articles about using this technique to mold optical surfaces on aluminum substrates for the fabrication of flats for use in military devices like tank periscopes and the like.
Problems: 1) The dimensional stability of the substrate isnít up to snuff, and the mirror looses its figure over time.
2) Shrinkage of the epoxy layer on the surface causes the piece to warp concave. This is more critical on a flat, though.
3) The epoxy itself creeps over time, and you get strange optical defects creeping in. Also, you have to protect the epoxy from moisture, as this will cause it to swell.
I actually own an Opticast mirror (a six incher). I have Foucault tested a number of times, and its figure has changed dramatically. It was pretty good, now it stinks. Im not sure if the problem is in the epoxy layer, or the plaster substrate, though I suspect the former. I have never actually built a telescope out of it, so I cant really say anything about how it performs in practice. I just got it so I could play with it. I would say that it would maybe pass for a low power scope, but thatís about it.
The idea of spin casting an optical surface has popped up many times on the telescope making lists I have seen. It generally ends in disappointment :(
Optical replication is really the only decent way to cheat traditional techniques and is within the realm of the average Joe. The idea of spin casting with a material that you can then polish to perfection is a good one, but no such material really exists. The only thing you could gain is not having to rough grind glass. Glass is king when it comes to polishability and dimensional stability, two qualities of extreme importance in a mirror.
There is a material called Electroless Nickel which is just nickel that has precipitated out of a solution. From what I have read, its polishability is outstanding. This is due to the fact that, like glass, it is an amorphous material (not crystalline, which metal normally is).
I have read on some promising success in depositing a layer of this material onto a surface (usually aluminum that was heat treated to anneal out stress) and polishing it to optical quality. The only drawback is that Electroless Nickel tends to have a lot of locked in internal stresses, and even just a thin layer can induce warping of the substrate.
At any rate, there still isnít a better way to make a truly fine mirror than good ole grindin'.
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